In the operation of prior art train vehicle speed control signal decoding apparatus, a fail-safe signal threshold determination is required. This has been provided in relation to base-emitter junction voltage drop of a common base transistor stage, with the output of same becoming an A.C. enable signal in conjunction with a fail-safe AND logic gate. It is desired that a predetermined minimum amount of input signal energy, in relation to the voltage and time product of that energy, be required before the train speed control system is enabled to become operative. This involves a transformation from the sensed input speed coded signal, which can be one of several different frequencies and different amplitudes, into an enable signal having predetermined characteristics.
A threshold signal detection operation requires a reference threshold to establish what input signal is above and desired as compared to what input signal is below and not desired in relation to that reference threshold. This reference can be obtained with a PN junction, such as the base emitter PN junction of an NPN transistor, used in a grounded common base mode driven by a transformer secondary, with no D.C. bias in the loop and no circulating D.C. currents. The A.C. input signal from the train vehicle track signal block passes through a set of band pass filters, to select the desired frequency input signals. A common carrier frequency signal is improved for each particular signal block and does not change, but the desired speed code frequency and phase modulation for each signal block can change. Six bit comma-free coded input signals, frequency shift keyed are used in this system, with phase shifts provided for clock purposes.
A synchronizing clock bit at 1/18 second intervals separates each bit by 180.degree. phase shifts, having all the same frequency for five bits and then a frequency shift in relation to the last bit of a particular code. The vehicle carried speed signal receiver detects each phase shift and each frequency shift. The involved waveforms are described in above referenced U.S. Pat. No. 3,810,161 of A. P. Sahasrabudhe entitled Apparatus For Receiving A Frequency and Phase Coded Vehicle Control Signal. The amplitude modulated output signals from the filters are similar to waveforms D and E shown in above referenced U.S. Pat. No. 3,551,889 of C. S. Miller.
A published article, entitled "Automatic Train Control Concepts Are Implemented by Modern Equipment" by R. C. Hoyler in the Westinghouse Engineer for September 1972 at pages 145 to 151, describes the automated train control system applied for the San Francisco Bay Area Rapid Transit (BART).